Problem: A circle with circumference $4\pi$ has an arc with a $225^\circ$ central angle. What is the length of the arc? ${4\pi}$ ${225^\circ}$ $\color{#DF0030}{\dfrac{5}{2}\pi}$
The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360^\circ} = \dfrac{s}{c}$ $\dfrac{225^\circ}{360^\circ} = \dfrac{s}{4\pi}$ $\dfrac{5}{8} = \dfrac{s}{4\pi}$ $\dfrac{5}{8} \times 4\pi = s$ $\dfrac{5}{2}\pi = s$